A computer-based sound synthesis and analysis system developed in the Section was used to present natural and synthetic vocal sequences to individual and group-housed squirrel monkeys. A significant role for individual experience was found, as shown by a differential response to familiar and unfamiliar stimuli. Similar findings were obtained in playback experiments involving common marmosits. Analysis of isolation call production in infant common marmosets indicates that species-typical characteristics are present from at least the first month of life. Structural variability gradually decreases over the first year, indicating a maturational course for this vocalization. A significant correlation was found between body weight and call duration in the immature marmosets.